The present invention relates generally to a hydromechanical system for limiting differential speed between differentially rotating members, such as in a vehicle drivetrain.
Known hydromechanical mechanisms have utilized hydraulic pumps which pump fluid in response to relative rotation between two rotating members for purposes of rotatably coupling the two rotating members. These systems generally include a hydraulic pump coupled to the two differentially rotating members, which in turn feeds a hydraulically actuated piston. The piston in turn acts on a clutch assembly connecting the two rotating members. The hydraulic pump provides volumetric flow of fluid that varies in direct proportion to the relative or differential rotational speeds of the rotating members. Generally, the hydraulic piston is equipped with an outlet orifice that restricts the outflow of fluid from the piston in order to generate a back pressure of fluid, which drives the piston to engage the clutch mechanism. Such systems therefore provide a capacity for torque transfer between the rotating members that varies in direct proportion to the relative or differential speed between the two shafts.
Accordingly, hydromechanical systems of the type described above require continuous relative rotation between the two members to produce torque transfer. This continuous rotation can produce undesirable levels of torque transfer at small differential rotational speeds. Such systems further do not have the capacity to arrest relative rotation between the members should the differential rotational speed exceed a desired or prespecified limit.
In vehicle drivetrains as an example, it is often desirable to limit differential speed between differentially rotating shafts. For example, four-wheel drive vehicles are becoming common, providing increased traction and safer operation of the vehicle. The four-wheel drive vehicle conventionally employs front and rear drive axles, having a front and rear differentials respectively for driving each wheel of the vehicle. A torque transfer case is generally used to distribute torque to the front and rear drive axles, and may be provided with an interaxle differential for dividing torque in a desired ratio. A selectively engagable clutch has been used to limit differential rotation between the front and rear axles of the vehicle, the clutch being operative to lock the interaxle differential upon sensing a predetermined differential rotation between front and rear output shafts of the transfer case. Activation of the clutch may be controlled by an electronic control system and associated speed sensors measuring speeds of the front and rear output shafts of the transfer case. It is also desirable to limit differential speed between other differentially rotating members in a vehicle drivetrain, such as in a differential associated with a drive axle of a vehicle. While an electronic control system can again be useful to limit differential speed between the differentially rotating members, in these examples as well as others, a simplified and less costly mechanical system, retaining advantages of this type of control system would be desirable.
As the need exists for systems capable of limiting relative rotational speed between two driveline components or other differentially rotating members, the present invention addresses this need without electronically controlled and actuated devices, in the form of an autonomous hydromechanical system which overcomes the problems of known hydromechanical mechanisms. The invention satisfies a need for a cost-effective system for limiting differential rotation between differentially rotating members.